Page 377 -

ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.35

Countercurrently regenerated demineralizers do not do well when there are significant
amounts of foulants in the feedwater. Suspended solids, iron, and organic content are the
primary culprits. Suspended solids accumulate in the resin beads. Countercurrently re-
generated units do not employ a full bed backwash as a normal portion of the regenera-
tion procedure. There is no means of purging the suspended solids from the bed on a rou-
tine basis. In fact, if a means was provided of doing so, this would interfere with the
countercurrent principle and would make a countercurrently regenerated unit operate less
effectively with lower quality, somewhere in between a coflow unit and a countercurrent
unit.
Iron and other types of foulants accumulate on resin beds and require periodic clean-
ing procedures. This is more of a disadvantage for countercurrently regenerated units as
it is generally more difficult to clean the resin in a vessel that has limited freeboard and
most countercurrently regenerated exchangers have limited freeboard. Any movement of
the resin bed, which is unavailable during a cleaning procedure, will impair quality over
several subsequent cycles.
Organics are less well purged from countercurrently regenerated units primarily be-
cause most of these units employ smaller chemical doses. Regeneration is the primary
means of purging the organics from the resin. When the regeneration is conducted with
a smaller chemical dose, it is less able to remove the organics. Therefore, at lower dose
levels the resin tends to become organically fouled more rapidly. The effects of organic
fouling are felt more strongly by countercurrently regenerated units because they tend to
make better water quality to start with.
The very best water quality in terms of resistivity that can be produced by a counter-
currently regenerated two-bed demineralizer is about 10 MO or 0.1/xmho. However, most
countercurrent systems do not do this well. The typical quality produced by a properly
designed and carefully operated countercurrent demineralizer is usually in the neighbor-
hood of 1 to 2 MO or 0.5 to 1/xmho. Many of the countercurrently regenerated systems
that are not perfectly designed operate in the quality range of 1 to 5/xmho or 200,000 kf~
to 1 MfL Countercurrently regenerated systems that are poorly designed or fouled may
produce worse quality than a coflow exchanger that is properly operated.
Coflow exchangers do not typically produce as high a water quality as a countercur-
rently regenerated unit. The typical reduction in conductivity from inlet to effluent in a
two-bed system is approximately 90%. The conductivity is caused by sodium hydroxide
due to sodium leakage from the cation vessel and hydroxide not neutralized after being
generated in the anion vessel. On a chemical equivalent basis, the sodium ion concentra-
tion is equal to the hydroxide ion concentration: Since sodium hydroxide has about twice
the equivalent conductivity of neutral salts such as sodium or calcium chloride, the
reduction in concentration across the two-bed unit is typically better than 95%, that is,
5% leakage or less.
Countercurrently regenerated units also produce lower silica leakage than do coflow
units. The best silica leakage that can be produced is in the range of 5 to 10 ppb. It is
more likely to see silica in the range of 20 to 50 ppb. Silica leakage depends on the ef-
fectiveness of the regeneration following the previous service cycle. Silica polymerizes
onto the anion resin, and the regeneration involves depolymerization. This process is de-
pendent on contact time and temperature. There is a potential problem in countercurrently
regenerated units with respect to silica in that the caustic dose is frequently chosen to op-
timize chemical efficiency. In some cases there may not be sufficient excess basicity to
depolymerize and dissolve the silica off of the anion resin. Therefore, when countercur-
rently regenerated units have a high percentage of silica in the influent, it is desirable to
avoid anion resin loading with more than 5 kgr/ft 3 of silica. In cases where the silica load
is 50% or greater of the total anion load, extraordinary regeneration measures may be
required.